1. Field of Invention
The invention relates to an automatic transmission. More specifically, it relates to a lock-up slip control for a hydraulic power transmission of the automatic transmission.
2. Description of Related Art
An automatic transmission has a lock-up clutch for preventing reduction of a transmission efficiency based on a fluid in a hydraulic power transmission of the automatic transmission. In this transmission, when the lock-up clutch is engaged during an engine rotation speed with the engine output power being low, a torque vibration of the engine is transmitted to the automatic transmission. In order to cutoff the engine vibration and improve fuel consumption, lock-up ON (i.e. the lock-up clutch is engaged) and lock-up OFF (i.e. the lock-up clutch is disengaged) are performed by a hydraulic control based on a lock-up diagram deciding a lock-up ON region and lock-up OFF region and an actual vehicle speed and an actual throttle opening of a driving vehicle. The lock-up ON region and the lock-up OFF region in the lock-up diagram correspond to a vehicle speed and a throttle opening, and the lock-up diagram is stored in a control device for the automatic transmission.
Recently, for the purpose of expanding the operation region of the lock-up clutch to low vehicle speed as much as possible and to improve fuel consumption, a proposal has been made that allows a slip control of the lock-up clutch for absorbing the torque vibration from the engine and supporting a torque transmission. An example of the technology is disclosed in Japanese Patent Laid-Open No. Hei8-28681. In this transmission, during the slip control, a feedforward control, which controls a hydraulic pressure applied to the lock-up clutch corresponding to a target value, is performed for bringing a slip rotation speed gradually to a target value. The slip rotation speed is a difference between an input rotation speed of the hydraulic power transmission, that is an engine output rotation speed, and an output rotation speed of the hydraulic power transmission, that is a transmission input rotation speed. During a lock-up clutch releasing state the slip rotation is brought gradually to a finalized slip rotation speed, which is called a basic target slip rotation speed in this specification. In order to prevent an overshoot of the control when approaching the basic target slip rotation speed as a result of a rapid change of the target slip rotation speed, a control is proposed in which an amount of reduction of the target slip rotation speed is reduced as the target value approaches the basic target slip rotation speed. Such a control is called a target value convergence control in this specification.
There are two cases for conversion from the lock-up OFF region to a slip control region. A first case is that the vehicle running condition based on the vehicle speed and the vehicle throttle opening change from the lock-up OFF region to the slip control region corresponding to an almost constant throttle opening and an increasing vehicle speed. A second case is that the vehicle running condition based on the vehicle speed and the vehicle throttle opening change from the lock-up OFF region to the slip control region corresponding to an almost constant vehicle speed and a rapidly changing throttle opening. In the first case, when the target value convergence control is performed immediately, there is no problem with controllability because the engine output rotation speed is stabilized. But in the second case, when the target value convergence control is performed immediately, the control is not stabilized because the engine output rotation speed is not stabilized. Then, in the above mentioned related art, the target slip rotation speed is set as the actual slip rotation speed and a feedforward value, that is a linear solenoid output value corresponding to the target slip rotation speed, is output for a predetermined time after satisfaction of conditions for the slip control. Then, the target value convergence control is not performed substantially. Therefore, problems of controllability are solved.
However, even if the control taught by the related art is performed, the following problems may occur during a time in which the engine rotation speed is unstable, such as right after moving to the slip control region.
(1) The actual slip rotation speed becomes unstable because of the instability of the engine rotation speed. Therefore, the control of the linear solenoid output value becomes unstable.
(2) When an engine output torque is reduced because of, for example, a drive at high altitudes or when the engine power output is otherwise lowered, an input torque against a throttle opening becomes small compared with a usual input torque against the throttle opening. Then, the linear solenoid output value corresponding to the throttle opening, which is on a table stored in advance, is set without consideration of the amount of the reduction of the engine output torque. Therefore, the linear solenoid output value becomes too large as compared with a proper value actually required at the lower level of power output. As a result, the lock-up clutch is further engaged, the actual slip rotation becomes small, and target slip rotation speed which is set based on the actual slip rotation value becomes small. Then, a further large linear solenoid output value which is not proper is output. Because of repetition of such states, the actual slip rotation speed is decreased largely before performing the target value convergence control, and the lock-up clutch may be engaged. As a result, a noise and a shock are produced thus making a driver feel uncomfortable.